The present invention relates generally to magnets, and more particularly to a shim assembly and to a method for making a shim assembly for reducing magnetic field inhomogeneity in a magnet, such as, but not limited to, a magnet having a pole piece.
Magnets include those having spaced-apart first and second pole pieces with generally opposing first and second pole faces, such as (but not limited to) "C"-shaped magnets. Some magnet applications, such as magnetic resonance imaging (MRI) for medical diagnostics, employ superconducting magnets to generate a high strength magnetic field within an imaging volume located between the pole pieces. The imaging volume must have a low magnetic field inhomogeneity for high quality imaging. Known techniques for reducing the magnetic field inhomogeneity within the imaging volume include using shimming rings attached to each pole face to reduce axisymmetric magnetic field inhomogeneity and using shims adhesively bonded to a shim tray attached to the shimming rings to reduce 3D (three-dimensional) magnetic field inhomogeneity (including non-axisymmetric inhomogeneity and any remaining axisymmetric inhomogeneity). The number, mass, and position of the shims on the pole face are determined by shimming techniques known to those skilled in the art. Adhesively bonding a shim to the shim tray is time consuming as the shim must be manually held in place, against the magnetic field, until the adhesive sets. Reshimming the magnet requires debonding the attached shim and cleaning the removed shim and the attachment area on the pole face.
U.S. Pat. No. 5,923,235 issued Jul. 13, 1999 and entitled "Shim Assembly for a Pole Piece of a Magnet" was invented by the same inventor as that of the present patent application and discloses an easily installed shim holder. The shim holder is inserted and rotated to attach it to a shim tray without the need for waiting for an adhesive bond to set. Each shim holder has holes into which shims are placed, as needed for shimming, typically by a press fit.
What is needed is a shim assembly for a magnet wherein the shim assembly provides for easier shimming of the magnet.